Concretes and mortars are cement compositions additionally containing aggregate (e.g., sand and rock) and water. When water is added to the cement, this forms a paste which hardens to a solid structure. Various additives have been used in these cement compositions to modify their properties for specialized applications. For example, long fibers such as asbestos reduce the sagging of these pastes and thus is beneficial when applying tiles to a vertical surface. Freezing point depressants are used when cements are to be poured in subfreezing temperatures. Cellulosic based polymers have been used in cements to control water retention, working time, and in certain cases, the sedimentation of particles in the pastes. Bentonite clay has also been used for this purpose. Other polymers, such as the polyvinyl alcohols and methyl methacrylates, have been used to reduce friction when pumping these pastes and to otherwise modify their workability. Fumed silica is used as an additive to make stronger concrete with reduced permeability.
The term "cement compositions" refers to hydraulic cements such as finely ground and calcined calcium silicates and calcium aluminates which when mixed with water react to form a hard, rock-like mass. There are many well known cement compositions such as: portland cement, portland pozzolan cement (containing about 15-40% pozzolan), blast furnace slag cement, slag cement (containing blast furnace slag and hydrated lime), masonry cement (e.g., adhesive mortars), construction concrete (containing sand and aggregate, oil-well cement (i.e., cements with retarders to prevent rapid setting so that they may be used at the high temperature and pressure environments of deep wells), aluminous cement (containing high amounts of calcium aluminates, expansive cements (containing high sulfate and alumina concentrations and which expand on hardening), air entrained cement (containing compounds which retain air bubbles and thus yield frost- and chemical-resistant concretes), lightweight concrete (containing low density materials such as furnace clinker, pumice, foamed slag, fly ash, gas, wood, etc.) heavy concrete (containing dense material such as barite, iron ore (i.e., ilmenite or hematite), steel, etc.), and low heat concrete (with modified compositions that minimize heat generation during the setting process).
With respect to oilfield cements, it is desirable, while drilling a subterranean well, to line the surface of the hole with hollow pipe known as casing. The casing is held in place by attaching the casing to the borehole wall with a cement slurry. The cement slurry is put in place by pumping the slurry down the inside of the casing to the bottom of the hole and up the annulus between the casing and borehole wall. The cement is then allowed to set for several hours to gain strength before any other operation is commenced.
The main purposes of primary cementing are:
1. Most importantly, to allow the segregation of individual formations behind the pipe so that fluids from one formation cannot flow into another, i.e., the cementation of an oil/gas well. This allows for the production from a specific zone. PA1 2. To add support for the casing by physically bracing or preventing the formation of pressure imposed on the casing. PA1 3. To retard corrosion by minimizing contact between the casing and corrosion formation waters. PA1 1. Reducing or increasing density; PA1 2. Increasing volume at reduced unit cost; PA1 3. Accelerating or retarding slurry thickening time; PA1 4. Increasing strength; PA1 5. Preventing loss of whole cement slurry; PA1 6. Increasing or improving the cement's durability; PA1 7. Decreasing water loss from the slurry; PA1 8. Increasing or decreasing the viscosity of the cement slurry; and PA1 9. Preventing gas migration.
Oilfield cements are similar to those used in construction (i.e., portland cement). The American Petroleum Institute has set specifications for oilfield cements. These as classified as "A" through "H", "J" and "N", all of which the present invention is useful in.
Cement additives in oilfield cements are materials mixed in the slurry for one or more of the following purposes:
The water loss of a "neat" cement slurry (cement and water only) is very high and rapid. When a slurry contacts a porous formation rock (such as an oil bearing sandstone) it may become quickly dehydrated by the water filtering into the formation. This causes the cement to "flash set." This may cause the casing to stick to the borehole before all the slurry is pumped in the annulus or before the casing is in the proper position.
Bentonite in concentrations of 0-14% (wt./wt. of dry cement) has been used in the past to control the water loss from the slurry. Cellulosic polymers such as carboxymethylhydroxyethyl cellulose (CMHEC) and hydroxyethyl cellulose (HEC) have also been used as water loss control agents and control the set of the cement at levels from about 0.2% to 0.9%.
Welan gum is an industrial grade of a bacterial polysaccharide produced by the growth of the Alcaligenes strain ATCC 31555 in a pure culture fermentation using carbohydrates as a carbon source. The product is recovered from the fermentation broth by precipitation with alcohol. Welan gum is a polysaccharide gum which comprises principally a heteropolysaccharide containing the neutral sugars D-glucose, D-glucuronic acid, L-rhamnose and L-mannose and glycosidically linked acetyl ester groups. The structure of this polysaccharide is described in Jansson P E, Linberg B, and Wildmalm G (1985) Carbohydrate Research 139, 217-223.
The rapidly hydrating welan gum composition used in the present invention is a novel combination of welan gum and a superplasticizer. U.S. Pat. No. 4,342,866, to Kang et al., describes a procedure for making welan gum and this is hereby incorporated by reference.
U.S. Pat. No. 4,981,520 to Hoskin et al., discloses a welan gum composition comprising welan gum, a phenolic resin such as resorcinal and an aldehyde. The compositions form stable gels in low salinity brines and are useful in selectively plugging highly permeable zones in a subterranean formation such as oil wells. This improves sweep efficiency during fluid flood oil recovery processes.
U.S. Pat. No. 4,963,668 to Allen et al., teaches the use of a low viscosity welan gum in cement compositions that allegedly exhibit improved workability, suspension of aggregates flow characteristics and resistance to water loss. Preferably, the range of amounts of welan gum used is from 0.1-0.5%.
U.S. Pat. No. 5,004,506 also to Allen et al., teaches welan gum in cement compositions as before, wherein a dispersant such as sodium citrate, sodium naphthalene sulfonates and the like, is added to reduce the viscosity of cement slurries and to serve as an aid in fluid loss control by dispersing the particles in the slurry.
U.S. Pat. No. 5,175,277 to Rakitsky et al., teaches and claims a rapidly hydratable welan gum which can be mixed with a carrier that is usually a superplasticizer, i.e., a dispersant. This is added to a dry cement/water premix and improves the cements workability, its fluid retention and prevents settling.
Finally, U.S. Pat. No. 5,290,768 teaches and claims a welan gum composition comprising welan gum and ethylene glycol. The composition displays unique viscosity and thermal properties which makes it an excellent insulation material.
None of the cited prior art however, discloses a stabilized suspension comprised of a hydrocolloid such as welan gum and a superplasticizer selected from the group consisting of sulfonated napthalene, sulfonated melamiine, modified lignosulfate, their derivatives and mixtures thereof. Moreover, none of the prior art discloses welan gum/superplasticizer compositions that are ground to an extremely fine particle size so as to afford its suspension superior viscosity and flow characteristics when used in cement, grout and oil field applications.